North Cascades Geology

The Basic Pattern Emerges

Early explorers in different parts of the North Cascades discovered such a variety of rocks and structures that the overall geologic pattern remained obscure for a long time. Even so, early workers recognized that the Earth's crust in the vicinity of the North Cascades is tilted up to the north. The tilted-up range was, and still is, higher in the north than in the south, and as a consequence, erosion has not only whittled the northern part down more rapidly and bitten deeper into the crust, revealing the older rocks, it has produced greater local relief.

Preservation of volcanic cover due to tilting of North Cascades up to the north.

In southern and central Washington, the Cascade Range is made of young volcanic rocks, ranging from 0 to about 40 million years old (Tertiary), erupted from many volcanoes of the Cascade Volcanic Arc . North of Snoqualmie Pass (Interstate 90), a foundation of older rocks begins to peek out from under this volcanic cover. Farther north, fewer young volcanic rocks are preserved and more of the foundation can be seen. In the area covered by this guide, the few patches of volcanic rocks that remain are mostly preserved in down-dropped fault blocks. All the rest of the volcanic cover has been eroded away, exposing the granitic rocks which once, as magma, invaded the foundation of older rocks and became the reservoirs of molten rock that fueled the volcanoes.
Something extra:An Early Geologist and Early Ideas

Material in this site has been adapted from a book,
Geology of the North Cascades: A Mountain Mosaic by R. Tabor
and R. Haugerud, of the USGS, with drawings by Anne Crowder. It is published
by The Mountaineers, Seattle.